The switching apparatus might be a closer for quick closing an electric circuit, in particular of the medium or high voltage level. Such a switching apparatus normally has two contacts movable to and from each other. Usually the movement is linear. The relative movement can either be achieved in that one of the contact parts is movable and the other stationary or in that both contact parts are movable, and each movable contact part is provided with an actuator unit.
Upon a triggering signal a driving means activates the actuating unit to close or break the current.
It is important that the movable contact part is held in its position with great stability, whether it is the open or closed position. This is achieved by the bistable mechanism. According to prior art the bistable mechanism is a toggle device. Examples of this are disclosed in WO 03/096502 and U.S. Pat. No. 6,538,347.
The toggle device used according to prior art consists of a number of rods connected to the actuating unit, which often is shaped as a tube. Each rod is connected to a piston urged by a compression spring radially towards the tube. In the two end positions the rods are inclined with respect to the radial direction with the inclination in different directions for the two end position. When the driving means moves the tube from one end position to the other, the connecting rods have to pass a middle position, in which the rods are radial and the springs are compressed at the maximum. Actuation of the tube, i.e. the actuating unit, from one end position to the other thus has the effect that the rod moves from one inclined position to the other via a radial middle position. Each spring thereby will be gradually compressed from the substantially uncompressed state at the end position until the rod reaches its middle position. Thereafter the spring is gradually released when the rod moves from the middle position to the other end position. This means that a force for compressing the springs has to be overcome for moving the actuating unit from either end position. Thereby the end positions become stable.
Although the known bistable mechanism normally functions satisfactory it has some drawbacks. The mechanism is complicated, expensive and also time consuming to assemble. The stroke for the actuating tube is maximized, and if a larger stroke is desired, the stroke length also of the spring pistons has to be increased for geometrical reasons for a given maximum inclination of the toggle rod. The complete mechanical housing thereby needs to be larger in diameter. This could be a problem because the insulator housing of a switching apparatus might be quite large as it is today, and a still larger insulator might be needed for a larger stroke.